The aftermarket gauges would need to use the sending units they are designed to work with. Unfortunately, that isn't going to be the same sending units as the factory gauges. If you don't want to hack your stuff up, you could get the pieces you need from a wrecking yard, and cut those up. Then run new wires for your new gauges, leaving the factory wiring untouched.
Or, you could get one of those three-gauge brackets and mount it someplace where you can see it. Maybe get all fancy and make a plexiglass glove box door and mount it in there.